Method of manufacturing an optical fibre, a preform and an optical fibre

ABSTRACT

A method of manufacturing an optical fibre, comprises: (i) forming a preform ( 10 ) for drawing into the fibre, the preform ( 10 ) comprising a bundle of elongate elements, ( 20,50 ) arranged to form a first region that becomes a cladding region of the fibre and a second region that becomes a core region of the fibre; (ii) drawing the preform ( 10 ) into the fibre. The bundle of elongate elements ( 20,50 ) comprises a plurality of elongate elements ( 20 ) of a lower purity dielectric material and at least one elongate element ( 50 ) of a higher purity dielectric material. The first region comprises a plurality of the lower purity elements ( 20 ) and the second region comprises the higher purity element ( 50 ).

This invention relates to the field of optical fibres.

Optical fibres are widely used in applications such astelecommunications. Such fibres are typically made entirely from solidmaterials such as glass, with each fibre having the same cross-sectionalstructure along its length. Transparent material in one part (usuallythe middle) of the cross-section has a higher refractive index thanmaterial in the rest of the cross-section and forms an optical core.Light is guided in the optical core by total internal reflection fromthe material surrounding the core, which forms a cladding region. Werefer to such a fibre as a conventional fibre or a standard fibre.

Most standard fibres are made from fused silica glass, incorporating acontrolled concentration of dopant, and have a circular outer boundarytypically of diameter 125 microns. Standard fibres can be single-mode ormultimode.

Applications often require very great lengths of optical fibre,typically tens or even hundreds of kilometres. Optical losses aretherefore a very important technical consideration, as even a small lossper metre can seriously attenuate signals that are guided over such longdistances in a fibre. Moreover, many applications requiring shorterlengths of fibre, such as fibre lasers, may be highly sensitive to loss.

One way of specifying the purity of glass is by its bubble class. Highpurity glass has a bubble class of 0 or 1. Another way is by its OH⁻content. High purity glass typically has an OH⁻ concentration of <10ppm.

Various techniques have been developed to produce standard optical fibreof high optical quality; such techniques are well known and includemodified chemical vapour deposition (MCVD), outside vapour deposition(OVD), vapour axial deposition (VAD) and plasma-enhanced chemical vapourdeposition (PECVD). Typical optical losses for fibres made by suchtechniques are around 0.2 dB/km at a wavelength of 1550 nm and around3-5 dB/km at a wavelength of 850 nm.

However, production of high-purity glass by prior-art techniques isdifficult, costly, and is specific to the production of a certain typeof preform.

In the past few years a non-standard type of optical fibre has beendemonstrated, called the photonic crystal fibre (PCF) [J. C. Knight etal., Optics Letters v. 21 p. 203]; such fibres have alternatively beencalled holey fibres or microstructure fibres. Typically, a PCF is madefrom a single solid material such as fused silica glass, within which isembedded an array of holes. Those ‘holes’ are usually air holes but mayalternatively be, for example, regions of a solid material (e.g. silicadoped with impurities to change its refractive index). The holes runparallel to the fibre axis and extend the full length of the fibre. Aregion of solid material between holes, larger than neighbouring suchregions, can act as a waveguiding fibre core. Light can be guided inthis core in a manner analogous to total-internal-reflection guiding instandard fibres. One way to provide such an enlarged solid region in afibre with an otherwise periodic array of holes is to omit one or moreholes from the structure. However, the array of holes need not beperiodic for total-internal-reflection guiding to take place (wenevertheless refer to such a fibre as a photonic-crystal fibre).

Another mechanism for guiding light in PCFs is based on photonic bandgapeffects rather than total internal reflection. For example, light can beconfined inside a hollow core (an enlarged air hole) by asuitably-designed array of smaller holes surrounding the core [R. F.Cregan et al., Science v. 285 p. 1537]. True guidance in a hollow coreis not possible at all in conventional fibres.

PCFs can be fabricated by stacking glass elements (rods and tubes) on amacroscopic scale to form a bundle having the required pattern andshape, and holding them in place while fusing them together. Thisprimary preform can then be drawn into a fibre, using the same type offibre-drawing tower that is used to draw standard fibre from astandard-fibre preform. The primary preform can, for example, be formedfrom fused silica elements with a diameter of about 0.8 mm.

Thus, the prior-art method of manufacturing PCFs having low lossrequires a large number of high-purity rods and tubes, each having arelatively small diameter compared with preforms for standard fibres.However, prior art methods of manufacturing high-purity glass rods arenot well suited to making high-purity tubes suitable for use in a PCFpreform. Furthermore, most manufacturers of high-purity glass are tooledfor making large boules of the glass and so making small rods requirescustom manufacturing runs and is therefore expensive.

An object of the invention is to provide an improved method ofmanufacture that enables production of low-loss PCF at a reduced cost.

According to the invention there is provided a method of manufacturingan optical fibre, comprising: (i) forming a preform for drawing into thefibre, the preform comprising a bundle of elongate elements arranged toform a first region that becomes a cladding region of the fibre and asecond region that becomes a core region of the fibre; (ii) drawing thepreform into the fibre, characterised in that (a) the bundle of elongateelements comprises a plurality of elongate elements of a lower puritydielectric material and at least one elongate element of a higher puritydielectric material and (b) the first region comprises a plurality ofthe lower purity elements and the second region comprises the higherpurity element.

Preferably, the higher purity dielectric material has a bubble class of0 or 1. The lower purity dielectric material then has a higher bubbleclass, for example 2 or higher. Preferably, the dielectric material isglass. Preferably, the higher purity glass has an OH⁻ content of <1 ppm.The lower purity glass may have an OH⁻ content of >10 ppm.

Because most of the energy in a guided lowest-order mode is concentratednear to the centre of that mode, it is possible for the fibre to exhibita low optical loss without it being necessary for all of the claddingregion and all of the core region to be made out of the higher puritymaterial; indeed, it is not even necessary for all of the core region tobe made out of the higher purity material. The cost and difficulty ofmaking low-loss fibre may thereby be reduced because the higher puritymaterial is only used in a part of the fibre.

The core region is considered to be the part of the fibre in which lightis guided rather than evanescent.

The second region may comprise a plurality of higher purity elements;thus a fibre with a larger higher purity core may be made. The firstregion may include at least one higher purity element. The first regionmay include a ring of higher purity elements that substantiallysurround, and are adjacent to, the second region.

The second region may include at least part of at least one of the lowerpurity elements, such that the core region of the drawn fibre includeslower purity material as well as higher purity material. The secondregion may include a plurality of the lower purity elements. The secondregion may include at least part of the elements forming an innermostring of lower purity elements that substantially surround, and areadjacent to, the higher purity element(s). It may be that the only partsof the elements forming the ring that are included in the second regionare the parts of the elements adjacent to the cane. The second regionmay include all of the ring.

Thus, although the mode may have an extensive cross-section, most of thelight energy is at the mode's centre, and it is the optical quality ofthe glass forming the part of the fibre core in which most of the lightis concentrated that is most significant in determining loss.

The elongate elements may have any suitable cross-section or anycombination of cross-sections for formation of the preform; for example,the elongate elements may be canes or tubes of circular or othercross-section. The tubes may be capillaries. The tubes may be largertubes that surround a plurality of canes or capillaries in the preform.

Preferably, the drawn fibre is a photonic crystal fibre such that thecladding region of the drawn fibre comprises a plurality of elongatebodies of a first refractive index embedded in a matrix material of asecond refractive index, different from the first. Preferably, the lowerpurity elements comprise an outer portion that forms the matrix materialand an inner portion that forms the elongate body, in the claddingregion of the drawn fibre. For example, the lower purity elements may bedielectric tubes, such that the inner portion is a hole.

In PCFs, the light will often be guided in a star-shaped lowest-ordermode that spreads into the part of the fibre in which holes are found(The holes in the fibre result from the holes in the tubes of thepreform).

Preferably, the higher purity element(s) is/are a cane or canes.

Also according to the invention there is provided a preform for drawinginto an optical fibre, the preform comprising a bundle of elongateelements arranged to form a first region that becomes a cladding regionof the fibre and a second region that becomes a core region of thefibre, characterised in that (a) the bundle of elongate elementscomprises a plurality of elongate elements of a lower purity dielectricmaterial and at least one elongate element of a higher purity dielectricmaterial and (b) the first region comprises a plurality of the lowerpurity elements and the second region comprises the higher purityelement.

Also according to the invention there is provided an optical fibre, thefibre comprising a cladding region and a core region, characterised inthat the cladding region comprises dielectric material of a lower purityand the core region comprises dielectric material of a higher purity.

Preferably, the cladding region comprises a plurality of elongate bodiesof a first refractive index, embedded in a matrix material of a secondrefractive index. Preferably, the elongate bodies are elongate holes.The core region may include material of a lower purity. The claddingregion may include material of a higher purity.

An embodiment of the invention will now be described, by way of exampleonly, with reference to the drawings, of which:

FIG. 1 is a perspective view of preform, according to the invention,formed of a bundle of tubes and canes;

FIG. 2 is a cross-sectional view of a fibre drawn from the preform ofFIG. 1;

FIG. 3 is an image of the intensity of light in a guided mode in amicrostructured fibre.

FIG. 4 is a perspective view of another preform according to theinvention.

FIG. 5 is a perspective view of another preform according to theinvention.

The preform of FIG. 1 comprises a bundle 10 of elongate silica tubes 20that are arranged in a triangular lattice pattern in the cross-sectionof the bundle. The bundle is held together by a silica jacket 40. Thesilica forming the tubes is HOQ310 from Heraeus QuartzTech Ltd., 1Craven Court, Canada Road, Byfleet, Surrey, KT14 7JL, which has a bubbleclass of 2 to 3 and 30 ppm OH⁻. At the centre of the bundle is anelongate cane 50 of solid silica. The silica forming the cane is glassmanufactured by the VAD process, which has a bubble class of 0 and <3ppm OH⁻.

The bundle 10 is drawn on a fibre drawing tower into fibre 110 (FIG. 2)in the same way as standard fibres are typically drawn from theirpreforms. In fibre 110, tubes 20 have fused to form an array of holes130 and silica matrix regions 120 (in which holes 130 are embedded). Atthe centre of the fibre 110, the translational symmetry of thetriangular lattice pattern of the holes 130 is broken. At the site ofthe defect that breaks the symmetry is region 150 (marked by a dottedline), which is formed of the high-purity glass of cane 50. The outerparts of fibre 140 are a silica jacket region 140 that does not containany holes and is derived from jacket 40. Silica regions 120, 140, 150have all fused to form a whole broken only by holes 130 and interstitialholes (not shown in FIG. 2) that result from imperfect tiling of thetubes 20 and cane 50 (which are of circular cross-section).

Light is guided in fibre 110 by a form of total internal reflection.Holes 130 reduce the effective refractive index of the parts of thefibre in which they are present. (The effective refractive index ofthose parts will be between the refractive index of the air in hole 130and the refractive index of silica regions 120; the exact value of theeffective refractive index depends upon the distribution of light in thefibre and can readily be calculated by methods known to those skilled inthe art.) As solid silica region 150 has a higher refractive index thanthe effective refractive index of regions containing holes 130, regionscontaining holes 130 act as a cladding region that confines light bytotal internal reflection to a core region in and around solid region150. It should be noted that the holes 130 are arranged on a trianglelattice as a result of their method of manufacture. There is norequirement for strict periodicity of holes in an index-guiding PCF.

The core region of fibre 140 is regarded as those parts of the fibre inwhich light is guided rather than evanescent. Light mode 260 in FIG. 3is typical of the guided mode of a fibre having the hole structure ofthe fibre of FIG. 2. Mode 260 substantially fills the solid silicaregion between innermost holes 230 and also spreads between those holes,forming a six-pointed star-like shape. The intensity of mode 260 is atits highest in the centre and most of the light energy is within theregion corresponding to higher-purity region 150 of the fibre of FIG. 2.However, the area of mode 260 also spreads considerably into regions220, corresponding to lower purity regions 120.

Because most of the light energy guided in the fibre of FIG. 2 is in thehigh-purity region 150, the loss seen by that light is low. The effectof spread into the lower purity regions 220 is not significant becauserelatively little light energy is guided in those regions, even though asignificant fraction of the cross-sectional area of the guided modespreads into those lower-purity regions. Thus a low-loss PCF 110 isprovided by the use of only one high purity element, cane 50. The costand difficulty of making fibre 110 is thus considerably less than thecost of making a similar fibre entirely from the higher-purity glass ofwhich cane 50 is comprised.

In an alternative embodiment (FIG. 4) preform 300 is similar to that ofFIG. 1 but the core region is drawn from a group of seven high-puritycanes 350, six of which replace the innermost ring of tubes 20. Tubes320 surround the seven canes and form a cladding region. Such anarrangement is useful for providing a PCF with a large core region forsupporting several modes.

In a further alternative embodiment (FIG. 5), preform 400 is similar tothat of FIG. 1, but an innermost ring of tubes 425 is also made of thehigher purity glass (in addition to cane 450). Tubes 420, which surroundtubes 425, are of the lower purity glass and form a higher-purity partof the cladding region in the drawn fibre. Such an arrangement ensuresthat a greater fraction of the guided mode propagates throughhigher-purity material.

1. A method of manufacturing an optical fibre, comprising: (i) forming apreform for drawing into the fibre, the preform comprising a bundle ofelongate elements arranged to form a first region that becomes acladding region of the fibre and a second region that becomes a coreregion of the fibre; (ii) drawing the preform into the fibre,characterised in that (a) the bundle of elongate elements comprises aplurality of elongate elements of a lower purity dielectric material andat least one elongate element of a higher purity dielectric material and(b) the first region comprises a plurality of the lower purity elementsand the second region comprises the higher purity element.
 2. A methodas claimed in claim 1, in which the second region comprises a pluralityof higher purity elements.
 3. A method as claimed in claim 1, in whichthe first region includes at least one higher purity element.
 4. Amethod as claimed in claim 3, in which the first region includes a ringof higher purity elements that substantially surround, and are adjacentto, the second region.
 5. A method as claimed in claim 1, in which thesecond region includes at least part of at least one of the lower purityelements, such that the core region of the drawn fibre includes lowerpurity material as well as higher purity material.
 6. A method asclaimed in claim 5, in which the second region includes at least part ofthe elements forming an innermost ring of lower purity elements thatsubstantially surround, and are adjacent to, the higher purity element(s).
 7. A method as claimed in claim 6, in which the only parts of theelements forming the ring that are included in the second region are theparts of the elements adjacent to the cane.
 8. A method as claimed inclaim 1, in which the drawn fibre is a photonic crystal fibre such thatthe cladding region of the drawn fibre comprises a plurality of elongatebodies of a first refractive index embedded in a matrix material of asecond refractive index, different from the first.
 9. A method asclaimed in claim 8, in which the lower purity elements comprise an outerportion that forms the matrix material and an inner portion that formsthe elongate body, in the cladding region of the drawn fibre.
 10. Amethod as claimed in claim 9, in which the lower purity elements aredielectric tubes, such that the inner portion is a hole.
 11. A method asclaimed in claim 1, in which the higher purity element (s) is/are a caneor canes.
 12. A preform for drawing into an optical fibre, the preformcomprising a bundle of elongate elements arranged to form a first regionthat becomes a cladding region of the fibre and a second region thatbecomes a core region of the fibre, characterised in that (a) the bundleof elongate elements comprises a plurality of elongate elements of alower purity dielectric material and at least one elongate element of ahigher purity dielectric material and (b) the first region comprises aplurality of the lower purity elements and the second region comprisesthe higher purity element.
 13. An optical fibre, the fibre comprising acladding region and a core region, characterised in that the claddingregion comprises dielectric material of a lower purity and the coreregion comprises dielectric material of a higher purity.
 14. A fibre asclaimed in claim 13, in which the cladding region comprises a pluralityof elongate bodies of a first refractive index, embedded in a matrixmaterial of a second refractive index.
 15. A fibre as claimed in claim14, in which the elongate bodies are elongate holes.
 16. A fibre asclaimed in claim 13, in which the core region includes material of alower purity.
 17. A fibre as claimed in claim 13, in which the claddingregion includes material of a higher purity.
 18. (canceled) 19.(canceled)